The use of threaded tubular connections for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid under pressure is well known. Oilfield tubular goods all use threaded connections for connecting adjacent sections of conduit or pipe. Examples of such threaded end connections designed for use on oilfield tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467, all of which are assigned to the assignee of the present invention.
In U.S. Pat. No. RE 30,647 by Blose, a particular thread form or structure is disclosed for a tubular connection that provides an unusually strong joint while controlling the stress and strain in connected pin and box members within acceptable levels. The pin member is equipped with at least one generally dovetail-shaped external thread whose width increases in one direction along the pin, while the box member is equipped with at least one matching generally dovetail-shaped internal thread whose width increases in the other direction. In this manner, the mating set of helical threads provide a wedge-like engagement of opposing pin and box flanks that limit the extent of relative rotation between the pin and box members, and define a forcible make-up condition that completes the connection. In this thread structure, the flank shoulder angles as well as the thread width can be used to control the stress and strain preload conditions induced in the pin and box members for a given make-up torque. Thus, by tailoring the thread structure to a particular application or use, the tubular connection or joint is limited only by the properties of the materials selected.
The wedge thread has been proven to be a reliable sealing mechanism in threaded connections. The dovetail, wedging action of the threads create sufficient interference in roots, crest, load flanks, and stab flanks to effect the thread seal. However, the thread diameter interference required to effect a pressure seal causes extreme tangential (hoop) stresses in the thin section of the box member.
U.S. Pat. No. RE 34,467 by Reeves discloses an improvement to the thread structure disclosed in the Blose reissue patent and specifically addresses the potential for false torque readings in the joint resulting from trapped thread lubricant in the clearance between the roots and crests of the threads. Reliance in the torque readings developed by the forcible make-up of the connection are necessary to insure that the design stress and strain preload conditions actually exist in the connection. Thus, the Reeves reissue patent discloses a thread structure whereby the box and pin threads are tapered, in addition to having thread widths that increase in opposite directions, so that the roots, crests, and flanks of the threads are moved into engagement as the joint is made up. The threads are particularly designed so that the complementary roots and crests move into engagement before both of the opposing stab and load flanks move into engagement, whereby the volume of lubricant in the clearance between the roots and crests is substantially reduced. In this manner, most of the thread lubricant is displaced to the helical clearance between the opposing load flanks and forms a long, very thin ribbon that has little if any effect on the proper make-up of the connection or the ability of the thread surfaces to form seals as they are moved together.
Because of imperfections in the machined thread surfaces that form the seals in a thread seal connection like the threads described in the Reeves reissue patent, thread lubricant can become isolated between sealing surfaces within the tubular connection. Once rotation between the pin and box members has advanced until the thread lubricant entirely fills the isolated volume between the pin and box members, additional rotation will produce an increase in the pressure of the lubricant. This increased pressure can result in higher tangential (hoop) and radial stresses in the connection, particularly in harsh cold weather environments, such as the North Sea, which cause the lubricant to become hardened and more viscous.
As shown in FIG. 1, prior art connection 10 includes a pin member 11 and a box member 12. Box member 12 has tapered, internal, generally dovetail-shaped thread structure 14 formed thereon and adapted for engaging a complementary tapered, external, generally dovetail-shaped thread structure 15 formed on pin member 11 to mechanically secure the box and pin members in a releasable manner.
Internal thread 14 of box member 12 has stab flanks 18, load flanks 16, roots 20, and crests 24. The thread increases in width progressively at a uniform rate in one direction substantially the entire helical length of thread 14. External thread 15 of pin member 11 has stab flanks 19, load flanks 17, roots 21, and crests 25. The thread increases in width progressively at a uniform rate in the other direction substantially the entire helical length of thread 15. The oppositely increasing thread widths and the taper of threads 14 and 15, cause the complementary flanks, roots, and crests of the respective threads to move into forcible engagement during rotational make-up of the connection and form sealing surfaces that resist the flow of fluids between the threads upon rotational make-up of the connection.
The pin member 11 or the box member 12 defines the longitudinal axis 13 of the made-up connection 10. The roots and crests of the box and pin members are flat and parallel to the longitudinal axis of the connection and have sufficient width to prevent any permanent deformation of the threads when the connection is made up.